Several methods are known to determine the state of charge of starting batteries in motor vehicles. Exemplary methods should be noted, which determine the state of charge via the measurable charge-proportional off-load voltage of a battery which has been idle for a longer period of time, typically four to five hours, using current integration and re-calibration. A method of this type is described in German Patent No. 35 20 985. Furthermore, model-based methods should be noted, which make it possible to determine the off-load voltage and therefore also the state of charge derivable from this off-load voltage, for a battery under load via adaptation of a model to the actual battery using the aid of sensor variables, such as battery voltage, battery current, and/or battery temperature.
The above-mentioned methods are relatively simple to implement; however, they result in errors during long operating phases of the vehicles, having relatively short or only few rest periods (e.g. taxi operations), because the state of charge may only seldom be re-calibrated or corrected via a measurement of the off-load voltage.
Model-based methods, in contrast, do not rely on rest periods for re-calibration, and the implementation of such methods, depending on the complexity of the underlying battery model, is relatively complex.